In the TCP client-server application, the server program must be running before the client, as the client will endeavour to create the connection with the server as soon as it is started. If the server fails to respond to the client when it opens this connection then the connection will fail, causing the client-server application to fail.

In the UDP client-server application, the server program need not be running before the client as the client will not attempt to create a connection as soon as it is started. Therefore is it possible to start the client application before the server and this will not cause anything to fail.